LINQ - SubmitChanges() does not update field in DB

Hi, I am trying LINQ for the first time,
and it is of course making problem to me :)
It does not give error, I google a lot to find a solution.
And my table (MS SQL 2008) has a primary key.

 

try
                        {
                            DataClasses1DataContext db = new DataClasses1DataContext(KLIS.Helpers.myConnections.GetConnectionString());
                            //var IPs = from p in db.T_MRO_IPs select p;

                            T_MRO_IP mro = db.T_MRO_IPs.Single(p => p.id == id);
                            mro.lastReply = DateTime.Now;
                            db.SubmitChanges();
                            //DataTable dt = KLIS.Helpers.myConnections.GetDataTableText("UPDATE T_MRO_IP SET lastReply=" + date.);
                        }

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